Geohydrology and Evaluation of Stream-Aquifer
Relations in the Apalachicola-Chattahoochee-Flint River Basin, Southeastern Alabama, Northwestern Florida, and Southwestern Georgia

ABSTRACT

The lower Apalachicola-Chattahoochee-Flint River Basin is underlain by Coastal Plain
sediments of pre-Cretaceous to Quaternary age
consisting of alternating units of sand, clay,
sandstone, dolomite, and limestone that gradually thicken and dip gently to the southeast. The
stream-aquifer system consists of carbonate
(limestone and dolomite) and elastic sediments,
which define the Upper Floridan aquifer and
Intermediate system, in hydraulic connection
with the principal rivers of the basin and other
surface-water features, natural and man made.

Separate digital models of the Upper Floridan aquifer and Intermediate system were con
structed by using the U.S. Geological Survey's
MODular Finite-Element model of two dimensional ground-water flow, based on conceptualizations of the stream-aquifer system, and
calibrated to drought conditions of October
1986. Sensitivity analyses performed on the
models indicated that aquifer hydraulic conductivity, lateral and vertical boundary flows, and
pumpage have a strong influence on ground-water levels. Simulated pumpage increases in
the Upper Floridan aquifer, primarily in the
Dougherty Plain physiographic district of Georgia, caused significant reductions in aquifer discharge to streams that eventually flow to Lake
Seminole and the Apalachicola River and Bay.
Simulated pumpage increases greater than 3
times the October 1986 rates caused drying of some stream reaches and parts of the Upper
Floridan aquifer in Georgia.

Water budgets prepared from simulation
results indicate that ground-water discharge to
streams and recharge by horizontal and vertical
flow are the principal mechanisms for moving
water through the flow system. The potential
for changes in ground-water quality is high in
areas where chemical constituents can be mobilized by these mechanisms. Less than 2 percent
of ground-water discharge to streams comes
from the Intermediate system; thus, it plays a
minor role in the hydrodynamics of the streamaquifer system.